Neurodegenerative disorders result when neurons normally do not reproduce or replace themselves, thus damaged neurons cannot be replaced. Progressive degeneration and/or death of neuronal cells often results in problems with movement (e.g., ataxias), or mental functioning (e.g., dementias). Many neurodegenerative disorders are currently considered to be incurable. Examples of neurodegenerative disorders include Parkinson's disease (“PD”), Alzheimer's disease (“AD”), and Huntington's disease (“HD”).
Parkinson's disease is characterized by a progressive degeneration of the dopaminergic pathway resulting in reduced concentration of the neurotransmitter dopamine in the brain which manifests itself as symptoms of slowness of movement (e.g., bradykinesia), rigidity, tremor and poor balance in the patient.
Biochemically, dopamine (3,4-dihydroxyphenethylamine) is formed by metabolism of dopamine precursors. For example, dopamine is formed by decarboxylation of the precursor levodopa (L-dopa; L-3,4-dihydroxyphenylalanine) through the enzyme aromatic L-amino acid decarboxylase (also known as DOPA decarboxylase (DDC)), both in the brain and in the peripheral circulation. Levodopa is in turn produced from the amino acid L-tyrosine by the enzyme tyrosine hydroxylase (TH).
Dopamine is metabolized to homovanillic acid (HVA) mainly through two metabolic pathways, namely (i) via 3,4-dihydroxyphenylacetic acid (DOPAC) by the enzymes monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT), and (ii) via 3-methoxytyramine by the enzymes catechol-O-methyltransferase (COMT) and monoamino oxidase (MAO).
The most common treatment of PD aims at restoring the dopamine concentration in the brain. Administration of dopamine is ineffective because it does not cross the blood-brain barrier. However, since the precursor levodopa does cross the blood-brain barrier, and is converted to dopamine in the brain, administration of levodopa has for a long time been, and still is, the drug of first choice for PD treatment.